Cisco Catalyst 2960 Series, Configuration Manual

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The system clock can provide time to these services:
• User show commands
• Logging and debugging messages
The system clock keeps track of time internally based on Coordinated Universal Time (UTC), also known as
Greenwich Mean Time (GMT). You can configure information about the local time zone and summer time
(daylight saving time) so that the time appears correctly for the local time zone.
The system clock keeps track of whether the time is authoritative or not (that is, whether it has been set by a
time source considered to be authoritative). If it is not authoritative, the time is available only for display
purposes and is not redistributed.
Real Time Clock
A real-time clock (RTC) keeps track of the current time on the switch. The switch is shipped to you with RTC
set to GMT time until you reconfigure clocking parameters.
The benefits of an RTC are:
• RTC is battery-powered.
• System time is retained during power outage and at system reboot.
The RTC and NTP clocks are integrated on the switch. When NTP is enabled, the RTC time is periodically
synchronized to the NTP clock to maintain accuracy.
Network Time Protocol
The NTP is designed to time-synchronize a network of devices. NTP runs over User Datagram Protocol
(UDP), which runs over IP. NTP is documented in RFC 1305.
An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic
clock attached to a time server. NTP then distributes this time across the network. NTP is extremely efficient;
no more than one packet per minute is necessary to synchronize two devices to within a millisecond of one
another.
NTP uses the concept of a stratum to describe how many NTP hops away a device is from an authoritative
time source. A stratum 1 time server has a radio or atomic clock directly attached, a stratum 2 time server
receives its time through NTP from a stratum 1 time server, and so on. A device running NTP automatically
chooses as its time source the device with the lowest stratum number with which it communicates through
NTP. This strategy effectively builds a self-organizing tree of NTP speakers.
NTP avoids synchronizing to a device whose time might not be accurate by never synchronizing to a device
that is not synchronized. NTP also compares the time reported by several devices and does not synchronize
to a device whose time is significantly different than the others, even if its stratum is lower.
The communications between devices running NTP (known as associations) are usually statically configured;
each device is given the IP address of all devices with which it should form associations. Accurate timekeeping
is possible by exchanging NTP messages between each pair of devices with an association. However, in a
LAN environment, NTP can be configured to use IP broadcast messages instead. This alternative reduces
configuration complexity because each device can simply be configured to send or receive broadcast messages.
However, in that case, information flow is one-way only.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
1524
Information About Administering the Switch